Parses a string representing an accelerator. The format looks like "<Control>a" or "<Shift><Alt>F1" or "<Release>z" (the last one is for key release). The parser is fairly liberal and allows lower or upper case, and also abbreviations such as "<Ctl>" and "<Ctrl>".
If the parse fails, accelerator_key and accelerator_mods will be set to 0 (zero).

Sets the modifiers that will be considered significant for keyboard accelerators. The default mod mask is Gdk::Window::CONTROL_MASK | Gdk::Window::SHIFT_MASK | Gdk:Window::MOD1_MASK, that is, Control, Shift, and Alt. Other modifiers will by default be ignored by Gtk:AccelGroup. You must include at least the three default modifiers in any value you pass to this method.
The default mod mask should be changed on application startup, before using any accelerator groups.